Problem: $ D = \left[\begin{array}{rrr}3 & -1 & 2\end{array}\right]$ What are the dimensions of matrix $ D$ ?
The first dimension is the number of rows in the matrix. How many rows does $ D$ have? The second dimension is the number of columns in the matrix. How many columns does $ D$ have? So, $ D$ is a 1 $\times$ 3 matrix.